The present invention is directed to the preparation of solid inorganic polymers having organo groups anchored to the surfaces of the polymers. The majority of the polymers formed are layered crystals which display intercalation activity.
The interface surfaces of solids are responsive regions of chemical and physical action. In many practical chemical and physical phenomena such as absorption, corrosion inhibition, heterogeneous catalysis, lubrication, ion exchange activity, adhesion and wetting, and electrochemistry, activity occurs as a consequence of the presence of a definable solid surface. Solids agents are preferred in most processes over solution or homogeneously dispersed reactive alternatives primarily because they greatly simplify efficient separation of products from reactants. However, solids invariably suffer from deficiencies in activity and selectivity in the conversions they effect, due to inherent heterogeneity in their active sites which arises from the nature of their surface structure. Furthermore, much of the active sites are usually buried within the surface, and as a result of these two factors, elevated temperature and low conversions are typically encountered. Exceptions in which homogeneous catalysts are employed have been the Monsanto process for the production of acetic acid from methanol and carbon monoxide employing rhodium, the production of linear alcohols from olefins and syngas, ethylene oxidation by the Wacker process, catalysis of olefins to form polymers, and other polymerization systems.
In an effort to achieve the best features of both homogeneous and heterogeneous processes, efforts have been made to chemically "anchor" known effective solution agents such as phosphines, nitriles, cyclopentadiene and the like, onto certain solids. Porous inorganic surfaces and insoluble organic polymers have been employed. Silica has been the inorganic of choice, the bonded ligand being attached by reaction with the --OH groups projecting from the surface. The organic polymer most used has been polystyrene, with an appropriate metal-coordinating function bonded via the phenyl rings. Results have been generally encouraging. However, there have been pervasive problems deriving from the non-uniform situation of sites which has manifested itself in loss of expected selectivity, activity and even in attrition.
Many inorganic solids crystallize with a layered structure and some could present sites for anchoring active groups. In this form, sheets or slabs with a thickness of from one to more than seven atomic diameters lie upon one another. With reference to FIG. 1, strong ionic or covalent bonds characterize the intrasheet structure, while relatively weak van der Waals or hydrogen bonding occurs between the interlamellar basal surfaces, in the direction perpendicular to their planes. Some of the better known examples are prototypal graphite, most clay minerals, and many metal halides and sulfides. A useful characteristic of such materials is the tendency to incorporate "guest" species in between the lamella.
In this process, designated "intercalation", the incoming guest molecules, as illustrated in FIG. 2, cleave the layers apart and occupy the region between them. The layers are left virtually intact, since the crystals simply swell in one dimension, i.e., perpendicular to the layers. If the tendency to intercalate is great, then the host layered crystal can be thought of as possessing an internal "super surface" in addition to its apparent surface. In fact, this potential surface will be greater than the actual surface by a factor of the number of lamella composing the crystal. This value is typically on the order of 10.sup.2 -10.sup.4. Although edge surface is practically insignificant compared to basal surface, it is critical in the rate of intercalation, since the inclusion process always occurs via the edges. This is because bonding within the sheets is strong, and therefore, basal penetration of the sheets is an unlikely route into the crystal.
Previous studies of the intercalative behavior of layered compounds have mainly been conducted by solid-state chemists interested in the bulk effects on the layered host materials. Graphite has, for example, been extensively studied from an electronic point of view. In general, the function of the host is essentially passive. That is, on intercalation the host serves as the matrix or surface with which the incoming guest molecules interact, but throughout the process and on deintercalation the guests undergo only minor perturbation.
In order for a more active process to occur during intercalation, such as selective complexation or catalytic conversion, specific groups must be present which effect such activity. There might also be some preferable geometric environment about each site, as well as some optimal site-site spacing. These considerations have not been extensively applied to intercalation chemistry simply because such kinds of active groups required are not found on layered surfaces.
An approach in which catalytically active agents have been intercalated into graphite or clays for subsequent conversions has been described in "Advanced Materials in Catalysis", Boersma, Academic Press, N.Y. (1977), Burton et al, editors, and "Catalysis in Organic Chemistry", Pinnavia, Academic Press, N.Y. (1977), G. V. Smith, editor, each incorporated herein by reference. In neither case could it be shown that any activity was occurring within the bulk of the solid. Rather, it is believed that edge sites are responsible for the reactivity observed. In none of the cases was the active site covalently anchored, or fixed upon the lamella of the host. Instead, the normal ionic or van der Waals forces of intercalated guests were operating.
One of the few layered compounds which have potential available sites is zirconium phosphate Zr(O.sub.3 POH).sub.2. It exists in both amorphous and crystalline forms which are known to be layered. In the layered structure, the site-site placement on the internal surfaces is about 5.3 .ANG., which leads to an estimated 25 .ANG..sup.2 area per site. This area can accomodate most of the functional groups desired to be attached to each site. The accepted structure, symbolized projection of a portion of a layer of this inorganic polymer and a representation of an edge view of two layers, are shown respectively in FIGS. 3, 4 and 5.
Besides the advantageous structural features of zirconium phosphate, the material is chemically and thermally stable, and non-toxic.
Quite a bit of work has been conducted on the zirconium phosphate, mainly because it has been found to be a promising inorganic cation exchanger for alkali, ammonium and actinide ions, Alberti, "Accounts of Chemistry Res." 11, 163, 1978, incorporated herein by reference. In addition, some limited work has been described on the reversible intercalation behavior of layered zirconium phosphate toward alcohols, acetone, dimethylformamide and amines, Yamaka and Koizuma, "Clay and Clay Minerals" 23, 477 (1975) and Michel and Weiss, "Z. Natur," 20, 1307 (1965) both incorporated herein by reference. S. Yamaka described the reaction of this solid with ethylene oxide, which does not simply incorporate between the layers as do the other organics, but rather was found to irreversibly react with the acidic hydroxyls to form a covalant bonded product, Yamaka, "Inorg. Chem" 15, 2811, (1976). This product is composed of a bilayer of anchored ethanolic groups aimed into interlayers. The initial layer-layer repeat distance is expanded from about 7.5 .ANG. to 15 .ANG., consistent with the double layer of organics present. The overall consequence of this reaction is to convert inorganic acid hydroxyls to bound organic alkanol groups. This conversion, while of interest, has limited if any improvement over the hydroxyls already available on zirconium phosphate.
Recent reports by Alberti et al, J. Inorg. Nucl. Chem. 40, 1113 (1978) and Maya, Inorg. Nucl. Chem. Letters, 15, 207 (1979) have described a method similar to that described herein for the preparation of similar layered compounds, and Maya has demonstrated usefulness as stationary phases or supports in chromatography.